#include <bits/stdc++.h>
#define rep(i,n) for(int i=(0);i<(n);i++)

using namespace std;

typedef long long ll;

template<class T> bool chmax(T &a, const T &b) { if (a<b) { a=b; return 1; } return 0; }
template<class T> bool chmin(T &a, const T &b) { if (a>b) { a=b; return 1; } return 0; }

int main(){
	cin.tie(0);
	ios::sync_with_stdio(false);

	int n;
	cin >> n;

	string s;
	cin >> s;

	int one = 0;
	int ans = 0;
	rep(i, n){
		if(s[i] == '3' || s[i] == '5' || s[i] == '7') ans++;
		if(s[i] == '1') one++;
		if(s[i] == '9'){
			if(one > 0){
				one--;
				ans++;
			}
		}
	}

	ans += one / 2;
	cout << ans << endl;
}